Polymer films having an electrical conductivity suitable for semiconductors may be formed from thiophene, 3-methylthiophene, 1-benzthiophene, aniline, and pyridine. Thin films of polymers prepared from thiophene and 3-methylthiophene in a high frequency (HF) discharge (13.56 MHz) have a semiconductor nature (σ about 10−7 to 10−10 Sm/cm) Semiconducting polymer films arranged on substrates made of quartz, glass or aluminum are obtained from halogen-substituted five-membered heterocyclic compounds, for example, 3-bromothiophene, and 2,2′-dibromothiophene as disclosed in Japan Patent No. 3-239721 A, published 1991. Polymer films having high electrical conductivity, however, cannot be obtained by these methods.
Thin polymer layers having higher electrical conductivity in the range of about 4×10−5 Sm/cm to about 10−1 Sm/cm and a method of preparing the thin polymer layers are disclosed in patent DE 4207422. The method consists of polymerizing compounds in a microwave discharge reactor. The compounds have a doping means chemically bound to the compound, for example, 2-iodothiophene, iodomethane or iodobenzene. The process is carried out under the discharge of a frequency from 0.1 GHz to 1000 GHz at a temperature of not more than 100° C. and pressure of from 10−3 mm to 10 mm Hg in the presence of a gas-carrier. Accordingly, the electrical conductivity of the polymer layers is increased by the use of compounds comprising doping agents bonded to the polymer.
Disadvantages of producing polymer layers by the above-described methodology include difficulty in controlling the complicated process which is based on several chemical reactions. For example, the conversion depends upon the properties of the layers to be formed and the corrosion activity of the initial iodine-containing substances in respect to the sophisticated reaction equipment.
Stability data of the electrical conductivity properties after heating are known only for semiconducting polymers prepared by chemical methods in a solution in the presence of doping agents other than iodine. For example, polymers of aniline and pyrrole are synthesized by oxidative chemical polymerization in the presence of anionic surfactants, such as sodium bis-2-ethylhexlyl sulfosuccinate, dodecylbenzenesulfonic acid, sodium salt of the acid or sodium dodecylsulfate. After heating to 175° C. and subsequent cooling to 20° C., with respect to polyaniline the electrical conductivity decreases by 2 to 3 orders of magnitude, and with respect to polypyrrole the electrical conductivity decreases by 4 to 7 orders of magnitude. An irreversible reduction of electrical conductivity after heating to 180° C. and subsequent cooling to 20° C. is also observed with respect to polyaniline films synthesized by an electrochemical method and doped with HCI or air moisture (measurements were carried out at a humidity in the range of about 10 to about 100%).
Semiconducting polymer films and methods for producing the films comprising the polymerization of mixtures of aniline and pyrrole with the addition of iodine in a high-frequency discharge (13.56 MHz) are also known. The electrical conductivity of the resultant films is in the range of about 10−9 Sm/cm to 10−8 Sm/cm, and changes depending on the air humidity when ranging from about 10% to about 70% humidity. When the air humidity increases to 92%, the electrical conductivity increases to 10−3 Sm/cm. When heated to 250° C., a film prepared from a mixture of aniline and pyrrole without the addition of iodine loses 15% of its weight, and a film prepared from the mixture with the addition of iodine loses 30% of its weight. The lower thermal stability of the iodine-containing copolymer may occur because iodine atoms more easily evaporate. These polymer films have several disadvantages such as low electrical conductivity at low air humidity and the dependence of electrical conductivity on humidity.
An increase in electrical conductivity of the films obtained by polymerization in a HF discharge at the same frequency (13.56 MHz) may be achieved by using monomers that are capable of forming polymers having a high content of conjugated double bonds. For example, unsaturated aliphatic or alicyclic compounds, such as acrylonitrile, 2-chloroacrylonitrile, 1,4-diazine, in the presence of a doping agent, such as iodine are disclosed in DE 3541721. The substrate temperature during polymerization may be in the range of about 15° C. to 250° C. The stability of electroconductive properties of the polymer films after heating may be presumed to be the same as for a polymer made from acrylonitrile, where the maximum temperature of stability of the electroconductive properties is not more than 200° C., since the polymer is destroyed between 220° C. to about 230° C. There are several disadvantages associated with these films such as insufficient electrical conductivity of the films (not more than 10−1 Sm/cm). Additionally, the presence of iodine vapors in the reaction chamber, which have a high corrosive activity, may cause damage to the sophisticated reaction apparatus. Furthermore, additional steps are required which involve a special gaseous catcher for radicals or halogen-containing hydrocarbons.
Electroconductive polymer layers such as films and coatings, prepared under conditions of a HF discharge do not exhibit a sufficient level of electrical conductivity and are significantly reduced after heating. This may be due to the participation of several different types of active particles in the process of polymerization with high frequency (HF) and ultrahigh frequency (UHF) discharges. Under such conditions, it is difficult to ensure the preparation of a layer structure, including the case of doped layers that is capable of obtaining high electrical conductivity and is stable after heating.
Additionally, electroconductive polymer layers and methods for preparing the layers where polymerization is carried out in a direct current discharge below atmospheric pressure and where the layers are formed on the cathode are disclosed in Russian patent No. RU 2205838. Here, the monomer may comprise organic compounds, for example, hydroxyl and/or amino-substituted aromatic quinones comprising at least two condensed cores, such as, 1-amino-9,10-anthraquinone. Polymer layers produced by this methodology have a sufficiently high electrical conductivity of about 10−2 Sm/cm. However, the resultant layers are limited by the specific temperature, specific time constraints and specific discharge current value required to perform the methodology.
Films having semiconductor properties and a method for their preparation by the polymerization of 1-benzthiophene with HF discharges (between a frequency of 10 kHz and 13.56 MHz) where the initial substance is evaporated by heating to 60° C. in the presence of a gas-carrier of argon or nitrogen and then doping with iodine is also known. The electrical conductivity of these films is in the range of about 10−4 Sm/cm to 10−5 Sm/cm. The major disadvantage associated with these films is their low electrical conductivity. Additionally, the methodology is very expensive due to the need to use an inert gas-carrier of oxygen or purified nitrogen.